perf data convert: Fix segfault when converting to json when cpu_desc isn't set

Arm64 doesn't have Model in /proc/cpuinfo and, thus, cpu_desc doesn't get
assigned.

Running
	$ perf data convert --to-json perf.data.json

ends up calling output_json_string() with NULL pointer, which causes a
segmentation fault.

Signed-off-by: Ilkka Koskinen <ilkka@os.amperecomputing.com>
Acked-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: James Clark <james.clark@arm.com>
Cc: Evgeny Pistun <kotborealis@awooo.ru>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20240223220458.15282-1-ilkka@os.amperecomputing.com